The present invention relates generally to apparatus for generating and detecting electromagnetic fields, and specifically to non-contact, electromagnetic methods and devices for tracking the position and orientation of an object.
Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
For example, U.S. Pat. No. 4,054,881, whose disclosure is incorporated herein by reference, describes a tracking system using three coils to generate electromagnetic fields in the vicinity of the object. The fields generated by these three coils are distinguished from one another by open loop multiplexing of time or frequency. The signal currents flowing in three orthogonal sensor coils are used to determine the object""s position, based on an iterative method of computation.
U.S. Pat. No. 5,391,199, whose disclosure is incorporated herein by reference, describes a system for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT patent application number PCT/US95/01103 and the corresponding U.S. national phase patent application Ser. No. 08/793,371, whose disclosures are incorporated herein by reference, describe a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of non-concentric sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates. The radiator coils operate simultaneously at different frequencies, for example at 1000, 2000 and 3000 Hz, respectively.
Other position sensing systems use a single coil in the catheter. The signals from the coil are sufficient, in conjunction with multiple field generator coils, to generate three dimensions of position and two dimensions of orientation information. The third dimension of orientation (typically rotation of the catheter about its longitudinal axis) can be inferred if needed from a comparison of the coordinates of two such sensors provided at mutually-spaced locations in the catheter and/or from mechanical information.
The above tracking systems rely on separation of position-responsive signals into frequency components, wherein each such component is assumed to correspond uniquely to a single radiator coil, in a known position, radiating in a narrow, well-defined frequency band. In practice, however, the radiator coils also generate magnetic fields at the frequencies outside the desired bands, for example due to mutual inductance effects. These mutually-induced fields lead to errors in determining the position of the object being tracked.
PCT patent application PCT/IL97/00100 and the above-mentioned U.S. patent application Ser. No. 09/125,544 describe apparatus for generating magnetic fields with reduced mutual inductance effects. The apparatus includes a plurality of radiator coils and driver circuitry coupled thereto, which drives the coils so as to generate magnetic fields at a plurality of driving frequencies. The radiator coils are configured and controlled so that each of them generates a field substantially only at a single, respective driving frequency. Preferably, circuitry is associated with at least one of the plurality of radiator coils for substantially eliminating magnetic fields generated by the at least one coil in response to fields generated by the other coils.
It is an object of some aspects of the present invention to provide improved electromagnetic radiator coils and driver circuitry therefor, for use in conjunction with object tracking systems in order to increase the accuracy of object tracking.
It is a further object of some aspects of the present invention to provide magnetic field generator coils and associated driver circuitry that generate fields having narrow bandwidths in the frequency domain.
In one aspect of the present invention, narrowed field bandwidth is achieved by canceling out mutual inductance effects among a plurality of coils, which generate magnetic fields at different frequencies.
In preferred embodiments of the present invention, a plurality of radiator coils generate magnetic fields at a plurality of different respective driving frequencies. Driver circuitry associated with the coils generates electrical driver currents therein, wherein for each coil the current comprises a major component at the coil""s respective driving frequency, and minor components at other frequencies. Typically, the minor components are substantially equal in amplitude and frequency and 180xc2x0 out of phase with parasitic currents induced in the coil due to magnetic fields generated by the other radiator coils, so as to substantially cancel the effect of the induced currents.
Preferably, the driver circuitry includes sensing apparatus, which measures the amplitude, frequency and phase of the induced parasitic currents in a the coil. The driver circuitry further includes an adaptive variable current supply, which generates the out-of-phase minor current components responsive to the amplitude, frequency and phase data measured by the sensing apparatus, so as to substantially cancel the effect of the induced parasitic currents. Most preferably, the process of measuring the parasitic currents and adjusting the minor current components is repeated iteratively until the amplitude of the parasitic components is reduced to below a predetermined threshold level.
There is therefore provided, in accordance with a preferred embodiment of the present invention, apparatus for generating magnetic fields, including:
a plurality of radiator coils; and
driver circuitry, coupled thereto, which drives the coils so as to generate magnetic fields at a plurality of driving frequencies, such that each of the coils is driven by a respective current that includes a basic current at a respective one of the driving frequencies and cancellation currents at one or more of the other driving frequencies, so that each of the radiator coils generates a field substantially only at the respective driving frequency.
Preferably, the cancellation currents are determined so as to substantially cancel parasitic magnetic fields generated by each of the coils in response to the magnetic fields generated by the other coils at the respective driving frequencies of the other coils. Most preferably, the cancellation currents are generated by the driver circuitry so as to be, in each of the coils, approximately equal in amplitude and opposite in phase to parasitic currents generated in the coil due to the magnetic fields generated by the other coils at the respective driving frequencies of the other coils.
In a preferred embodiment, the driver circuitry includes a current analyzer, which is coupled to measure parasitic currents flowing in each of the coils at the respective driving frequencies of the other coils, and wherein the driver circuitry is adapted to generate the cancellation currents responsive to the measured parasitic currents. Preferably, the driver circuitry includes an adaptive current supply, which is driven by the current analyzer to generate the cancellation currents. Alternatively or additionally, the driver circuitry includes a memory, which is adapted to store, for each of the coils, data indicative of a waveform of the respective current with which the coil is to be driven, and wherein the data are modified responsive to the measured parasitic currents. Most preferably, the driver circuitry is adapted to adjust the generated respective current in each coil so as to minimize the parasitic electrical current measured therein.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for generating a magnetic field having a desired narrow frequency band, including:
driving a radiator coil with an electrical current at a frequency in the desired narrow frequency band;
analyzing the current flowing in the coil to measure induced current components at one or more undesired frequencies; and
adjusting the current driving the coil so as to include correction current components at the one or more undesired frequencies, responsive to the measured induced current components, so that the coil generates a field substantially only at the respective driving frequency.
There is additionally provided, in accordance with a preferred embodiment of the present invention, a method for tracking an object, including:
driving a plurality of radiator coils with electrical currents at respective driving frequencies so as to generate a plurality of magnetic fields at the frequencies in a vicinity of an object being tracked;
analyzing the currents flowing in the coils so as to measure, in each of the coils, induced current components at one or more of the frequencies of the other coils;
adaptively adjusting the current driving each of the coils so as to include correction current components at the frequencies of the other coils, responsive to the measured induced current components, so that each of the coils generates a field substantially only at the respective driving frequency;
placing one or more sensor coils at a locatable site on the object; receiving signals generated by the sensor coils in response to the magnetic fields; and
analyzing the signals to determine position and orientation coordinates of the object.
There is further provided, in accordance with a preferred embodiment of the present invention, an object tracking system, including:
a plurality of radiator coils; and
driver circuitry, coupled to drive the coils to generate magnetic fields at a plurality of driving frequencies, such that each of the coils is driven by a respective current that includes a basic current at a respective one of the driving frequencies and cancellation currents at one or more of the other driving frequencies, so that each of the coils generates a field substantially only at the respective driving frequency;
one or more sensor coils, fixed to the object; and
processing circuitry, coupled to receive and analyze currents induced in the sensor coils due to the magnetic fields, so as to determine coordinates of the object. The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which: